1. Field of the Invention
The present invention relates to a stencil printing ink and, in particular, to a stencil printing ink that is suitable for use in a rotary digital stencil printing machine.
2. Description of Related Art
The stencil printing process, unlike printing processes such as offset printing, gravure printing, and relief printing, has ease of operation and simplicity since there is no necessity for complicated operations such as washing after use and no necessity for a specialist operator. In accordance with the use of a heat sensitive plate-making method employing a thermal head as a device, in the stencil printing process, image processing is digitized, high quality printed material can be obtained simply in a short period of time, and its convenience as an information processing terminal is therefore widely acknowledged.
Rotary stencil printing machines in which plate making/plate mounting/plate ejecting operations for a stencil sheet (master), an ink supply operation, a printing operation, etc. are automated are known as digital stencil printing machines, etc. and are widely used in offices, schools, etc.
As the stencil printing ink, a water-in-oil (W/O) type emulsion ink has been generally used conventionally. W/O type emulsion inks have the function of suppressing changes in ink composition and physical properties even if the ink within the printing machine is in contact with the atmosphere when the printing machine is allowed to stand in a nonoperating state. That is, water, which is an inner phase component of the emulsion ink, is covered by oil, which is an outer phase component, and evaporation of water is therefore suppressed.
It is thought that drying of ink on a printed material printed with a W/O type emulsion ink proceeds as a result of the ink penetrating between paper fibers of printing paper, which is a substrate (print medium), and the emulsion being gradually separated into an oil phase and a water phase by contact with the paper fibers, and water, which is a main component of the ink, evaporating by contact with the atmosphere. However, since water in the ink transferred to the substrate cannot come into contact with the atmosphere for a short period of time subsequent to printing, the drying properties immediately after printing rely on drying by penetration, and since the viscosity of the W/O type emulsion ink is designed to be relatively high, the penetration speed is not high, and it cannot be said that the ink drying properties immediately after printing are adequate.
Speeding up the drying of a printed material is very important in stencil printing. Unless the printed material is dry, an operator cannot handle the printed material, and the advantage of stencil printing of ‘obtaining high quality printed material in a short period of time’ cannot be fully exploited.
Various improvements have been carried out in order to improve the drying properties on printed materials; for example, an ultraviolet curing type ink for stencil printing that is dried and fixed by irradiation with ultraviolet rays (Japanese Patent Laid-Open Publication No. 2002-30238) is known. Furthermore, a water-based ink for stencil printing has been developed from the viewpoint of environmental protection and safety, and a stencil printing method in which penetration of a water-based ink into paper is enhanced by adding a base to a printed surface immediately after printing is known (Japanese Patent Laid-Open Publication No. 2001-302955).
However, when a drying method involving a chemical reaction is used, it is necessary to employ irradiation equipment for the curing energy, coating equipment for a reaction solution, energy therefor, etc., and it is also necessary for the ink to contain expensive starting materials.
In stencil printing, the ink is transferred onto the surface of a printing paper by passing through perforations of a stencil sheet by means of printing pressure caused when the stencil sheet and the printing paper are pressed against each other, and if the viscosity of the ink is decreased in order to increase the speed of penetration of the ink into the printing paper, the ink passes easily through the perforations, the volume of ink transferred becomes excessive, fine characters, fine lines, etc. therefore spread, and as a result there are the problems that a detailed image cannot be obtained and the drying properties become poor.
It is therefore an object of the present invention to provide a stencil printing ink that enables the volume of ink transferred to a substrate to be controlled regardless of the level of viscosity of the ink, thereby enabling a detailed image to be printed, and that enables the drying properties on printed material to be improved without using special means, equipment, energy, etc. such as the application of heat, light, or a reactive material; and a stencil printing method employing the stencil printing ink.